The problem is your system memory AND your video card.
Vista needs 1 Gb to operate correctly and will leave you about 300 Mb memory for running programs.. now take your Vid card.. it will take about half of that 300 MB to utilize for games, plus the Graphic Processor is going to steal CPU clock cycles to render the game.
You need to get a Vid Card, preferably NVidia (Radeons have trouble with Vista with drivers and games / software) ATI/AMD (same company) does not support older hardware and their drivers are quite buggy (Trust me.. My current system is running a Radeon X1300 All-In-Wonder 2006 with 512 MB memory.. Vista only recognizes 256 Vid memory because of AMD's Drivers, and the TV Tuner is not supported by AMD on a Vista system, and the card is barely over a year old.. that is AMD support for you) I have to find work arounds just to get the crappy and fuzzy TV Tuner in this card to work.. that only took about 3 weeks. All my games have to run on Med low settings or just plain Low settings.. Radeon sucks for Vista.. at least the product I have does.. No AMD support except faulty drivers (as usual AMD), half Vid memory recognized, and TV Tuner not supported at all by AMD and AMD's refusal to support the card, at least on Vista systems.. and the card is only 1 year and 3 months old.. Bought Dec 2006..
My new system is going to be 100% Vista compliant.. see my thread on Finalizing my System Build..
GeForce 8800 GTX OC2 756Mb gDDR3 memory.. the best card on the market, next to the water cooled Ultra (only 6% difference in performance on 2 tests.. all other tests the OC2 I have is faster)
I will have no Vid memory problems.. No GPU problems, and unlike AMD/ATI Radeon, the GeForce has built in Physics engine and real support.. even the older cards are supported if installed on Vista.. all the way back to the 6600 series, only the 8600 and newer cards are DirectX 10 compliant)..
in the affordable range, an NVidia GeForce 8800 GTS with 512 MB memory would do wonders.. it too has built in Physics engine
As for system memory, I have 4 GB on Vista Home Premium 32 bit.. It is not recommended to game with Vista if your system memory is less than 2 GB.. that is about 700 MB for system to operate (loads 700 MB, utilizes 300 MB) and 1.3Gb to operate programs.. Also using the Vista Ready Boost for your Virtual Drive (at least a 2 Gb USB Memory Stick ..$14.99 for Sandisk at Wal-Mart .. in the Photo Clearance section) will improve your stability and memory usage with your low ram ammount for the time being, but it will not help your games, but it will lower your hard disk access quite a bit (read /write virtual memory).
Honestly, and my AMD rant asside, You should have at least 2 Gb system memory when running Vista, and put in a real vid card that does not use shared memory.. and with todays games.. the vid card should have a minimum of 512 Mb on board.